[CPA Nerd]

I planted grass seed back in late April/early May. The baby grass is doing well, but I do have some weeds in the lawn, especially since I did not apply a pre-emergent. I plan on doing another overseed in September.

Will applying weed killer to the entire lawn now do any harm to the new grass? Will it prevent new grass from growing in the fall?

 

[Greenrebellion]

Depends on what grass. Most herbicide labels will tell you when you are able to apply to new grass safely. Most labels will also tell you how long you need to wait to overseed after applying the product.

 

[jessehurlburt]

It should be fine at this point, but def double check the label. Usually a month or 4 mows. After applying a post emergent herbicide you typically need to wait a month before seeding (again check the label). It won't interfere with a fall overseeding. If you were applying dimension, or some other pre-emergent, then it could impact your seeding, but you're trying to kill actively growing weeds, so that wouldn't apply. Are you spraying Weed b gone or using a weed and feed granular? What types of weeds do you have?

 

[ForsheeMS]

It also depends on your current temps and the herbicide being used. Hot weather and a lot of herbicides are tough on established grass, much less tender new grass.

 

[OnyxsLawn]

I've been using WBG at 6-8 oz/k with no issue on PRG and TTTF planted in early may for the last 2 weeks. I dropped to 4 oz/k since the temps were so high this week. Assuming you're experiencing the same cool off we have right now you should be safe to apply. If you're really worried you can always go half app or pull the weeds by hand.

 

[CPA Nerd]

It should be fine at this point, but def double check the label. Usually a month or 4 mows. After applying a post emergent herbicide you typically need to wait a month before seeding (again check the label). It won't interfere with a fall overseeding. If you were applying dimension, or some other pre-emergent, then it could impact your seeding, but you're trying to kill actively growing weeds, so that wouldn't apply. Are you spraying Weed b gone or using a weed and feed granular? What types of weeds do you have?

I am planning on spraying Ortho Weed B Gon. I have several different types of weeds. Some crabgrass, clover, broadleaf weeds.